Integral heating elements

ABSTRACT

Combinations are provided of organic binder, metallic particles, or powder converting to a metal under mild reducing conditions, and porcelain enamel frit, preferably in a range of sizes, at least a part of which has average particle diameters which are at least 5 times the diameters of ultimate metal particles. These combinations are used to apply integral heating elements to various utensils.

D United States Patent [15] 3,640,764 Shevlin 1 Feb. 8, 1972 [54] INTEGRAL HEATING ELEMENTS 3,149,002 9/1964 Place at al ..252/514 X 3,396,055 8/1968 Hedden et al... .....252/5 14 X [721 lnvenm" Shevlin Lake 3,440,182 4/1969 Hoffman ..l06/48 x [73] Assignee: Minnesota Mining and Manufacturing Primary Examiner-Ralph S. Kendall Company, Saint Paul, Min Anomey-JGnney, Alexander, Sell, Steldt & Delahunt 22 F1 d: Se t. 26 1968 1 57 ABSTRACT [21] Appl. No.: 762,912 v Combinations are provided of organic binder, metallic particles, or powder converting to a metal under mild reducing [52] US. Cl ..l17/2l72,1ll076/24189, l2l572/25l conditions, and porcelain enamel mt preferably in a range of 5 l t "7/21 I I18 sizes, at least a part of which has average particle diameters 218 215 which are at least 5 times the diameters of ultimate metal Parlsl ticles. These combinations are used to apply integral heating elements to various utensils. [56] Reierences Cited m Claims 1 Drawingfigure 2,942,992 6/1960 Dumesnil uuman STATES PATENTS ($2,255? A 40 B 60 l I 60 mgz FRI? :2 a. 50100, Can w INTEGRALHEATING' ELEMENTS This invention :relates to new combinations useful in producing resistance element or integralheating elements and to processes v.ancillary to the use. of such combinations'in producing .electrically heated dishes, vessels, utensils, etc. in particular-this invention relates to the novel combination of porcelainenamel frits and silver oxide or noble metal'particles with a fugitive noncarbonizing organic binder in which at least .part-of the-.porcelainenamel frit is larger byat least 5 times, e.g., 5 to times than the conductive particles.

'l-lereand .elsewheresilver oxide is considered :in particular. lt must be recognized that, on firing, silver oxide is converted to silver metal in finelydivided form. It will be understood: that other conductive metals are generallysimilar when of comparable particle size except as for the necessity for'reduction. Silver oxide'provides very'fine particles of silver of the order of less than 1' micron in diameter.

It is well known in the art to combine metal powders and glass frit. Such compositions are commercially available in proportions adapted .to give conductive films or thick film conductors,'resistances,etc. Such compositions are generally of value over very narrow changes in composition i.e.,a very small percentage change effectively renders a composition either excessively conductive or substantially very' highly. electrically nonconductive. These compositions moreoverprovide narrow electricalpaths like wires sothat use thereoffor heating shells limits the area of the sheet to whichheat can be supplied.

.It has been found that compositions prepared'to contain silver oxideparticlesandporcelain enamel frit'particles-in-a rangeof larger sizes are adaptable to relatively wide variation in compositionwhile providing resistancefilms whichmay'be firedasan integral part of a vessel or utensil, for example, for serving food. These compositions and their usesthus fulfill objects of the invention.

It is 'found .that the frit particles should be of a sizerange of which a substantial part is five or more times the average size of the metal (i.e., Ag 0) particles. It is possiblefor a smallportion of the frit particles to be of the same magnitude-asthe smallest metal .particles or even smaller. :In the following discussion some speculation is made as to the reason that such novel compositions are so effective but the reasons are noten- 'tirely clear. Thus, it is found that,.after firing, the, porcelain enamel matrix can be dissolvedaway from the embedded silver and a frangible reticulumis obtained. The reticulum appears to be composed of innumerable rninute particles of silver which. contact one another but are not generally fused into relatively massive-wires or sintered together.

Because of the multiplicity of contacts inthe networkthe heating elements obtained arethus less subject to mechanical or electrical breakdown because breaking of even several contacts does not throw so much load on-parallel contacts as -:to cause melting and breakdown with failureof the'heatingelement. Instead the load isslightly increased over each contact. it is only when a substantial number of @the contacts .are severed that the electrical load causes breakdown. Such a situationmay occur wherecontact is made to a'heating element. Arthe junction there maybe relatively high current density and atendency to burn out connections. This is-overcome by providing graded contacts in which a metal ric'her composition'is applied at points of contact. This provides-innumerable contacts -through a network and distributes the electrical loadto the heating element.

Reverting to the compositions employed in these novel heating elements, the compositions are illustrated by a drawing, which is a three-component diagram-in'percents by weight of various compositions fabricated into 'heating elements of the same dimension by the procedures hereinafterdescribed.

Separate coarse and fine frit components are usediforconvenience in illustrating more clearly the'effects ofvariations in the proportions of frit particles which are much coarser than the silver oxide particles. It will be seen that generally even broader ranges of compositions are effective using the :relatively 'coarsefrit'than-usingthe finer frit. The finer frit is,

"nonetheless, coarse'incomparison to the silver oxide. The usual combinations-of theprior art are so prepared that the metallic component is very likely to'bein the size range of the coarsest particles.

The preferredcompositions of the invention are those lying in-the-general area ABCD away-from extreme edges AB and CD, that is those which are neither all'coarse or all fine but are averageand more similar to asimple milled frit. In this figure the line'AD represents compositions having essentially infinite resistance andline .BC represents compositions having approximately lohm resistance in the size of conductor illus- -trated.'lf.-sufficient points were determined it is quite possible :that'these'lines might bow or bulge to some extent. It will be obviousthat'such a'graphishighly specific and that other ing. Thus it'wilkbe evident fromthe graph'that broader ranges of composition are useful than has been general heretofore xusing low temperature glassyfrits milled to being generally as -"fine' as or finer than the metal particles. This appears to be an advantage 1 of usingcoarser frits and recrystallizing porcelain enamels.

lln 'the'drawing, the apicesare designated as -l00 percent of the respective components 'the increasing percentages of whichare indexed'along 'theadjacent'sides. Each apex is, of course, also 0 percent of both other components. The various points'representcompositions which are marked with-the resistance'in ohms-of an element about cm. long, about 6 mm. wideand'about 0.125 mm. thick produced as described "hereinafter. These-resistances are termed specific element resistances'for convenience. lt'willbe-seenthat the specific element resistancesincrease rapidly at lo'wercontents of silver and decrease-above S0-60percent silver'to' the point of being excessivelyconductive. It 'will beevident that any desired combination of properties can 'be attained within these teachingsusinggreater or lesser amounts of porcelain enamel frits dependingonthe resistance, thickness, width and length of conductor desired.

Thesilver oxide used for these compositions, and generally use'fulandgpreferredin such'compositions averages around 1 micron -in particle size withismaller particles as low as 0.5 .micronand-occasionalparticles up to about 2'microns. Such materialisreadily availablein chemicallypure grades.

The preferred matrix'material is aporcelain enamel which is =compatible-with'the base to 'which'the heating element is'to be attached. A commonly 'availableand generally useful enamel is 'a moderate phosphate content enamel containing about l5- 20:percent-oftitania. A typical such composition is:

X: by weight Dehydrated borax 19.] Quartz 42.0

- Sodium nitrate 7.8 Sodium silico'fluoride L2 Potassium silicofluoridc 7.8 Anhydrous sodium monophosphate 3.2 Titania I89 So far as is :known the composition as such is not critical although'it ischaracteristic thatthecompositions recrystallize and mature at alow enough temperature so that the metal does notcoalescer-and flow together". Thus,the enamel-is nonfluxin'gfor the metal. lt isat least possible 'that'recrystallization of the 'titania opacifier may decrease flow. In any event, it is preferredithat a porcelain enamel-frit be'used'in compositions forproducing'heaterelements on metal utensils.

As obtained, frits are usually obtained as glassy flakes or granules which must be milled and then give distribution of particle sizes. In order for compositions of the invention to be effective the particles of the milled frit must be predominantly usually 80-85 percent or more, larger by a factor of 5 or more than the average size of the metal particles. The frit may be milled by any convenient method. It may be convenient to screen the coarsest particles from the frit and then further mill them to smaller particles. In the compositions used in construction the drawing, the coarse particles passed 150 mesh and were held on 270 mesh, that is they were between 53 and 105 microns in diameter. The frit used for the fine component was extensively milled (in ethanol for 5 hours or longer) to be of a size range from less than 1 micron to 5 microns with some particles up to microns.

A particularly useful embodiment of the invention is in transfer sheets of frit which can be applied to a vessel and fired in place. This offers some valuable conveniences in fabrication. In this embodiment, frit of suitable particle size and silver oxide are combined with an organic binder which decomposes on heating without charring or coking and a solvent for the binder. A small amount of plasticizer may be included if desired. The thick composition is cast or knife-coated to a desired thickness e.g., 0.05 to 3 mm. on a smooth support such as glass, polished metal, silicone or polyethylene impregnated paper or a polymeric film. The solvent is then evaporated. Naturally, because of the valuable metal contents of these compositions thinner films are preferred but it should be noted that a much lower metal content may be useful in a relatively thicker film. Thinner films down to about 0.01 mm. are conveniently prepared directly on a utensil or for transfer from backings using silk-screening processes.

in this embodiment it appears that the range of particle sizes of frit helps to entrap the silver particles so that they do not segregate or settle under the influence of gravity during drying. The binder also retards segregation of silver particles so that a homogeneous suspension is retained. For example, the compositions used in constructing the drawing were prepared from 100 grams of the dry combination in the various proportions as shown in the figure of frits and silver oxide suspended in a solution of grams of polyisobutylene (available as Vistanex L-120 having average molecular weight 120,000) in 50 grams of toluene. Other binders may also be used as alternatives or other solvents. The composition is knife-coated t about 0.6 mm. on a silicone-treated paper support sheet and air-dried. Deairing may be applied to the suspension before the coating operation but is not necessary in most instances. Entrapment of air bubbles in porcelain enamels is frequently considered to provide a measure of stress relief.

After drying the film or tape of the composition may be rolled upon itself into rolls for storage and later use or it may be used directly.

It is convenient to cut the tape to sheets, for example 7.5x12.5 cm., which fit a particular utensil or are adapted to provide a heating element of particular output. Naturally if the backing on which the film is cast is glass or metal, the film must be pealed away first. Alternatively, when cast on treated paper or polymer-film as above, the backing may be retained to give additional support and removed after transfer of the film to the utensil to which it is to be adhered.

The film, for example in the cut sheet, is converted to a serpentine or meandered heating element by making suitable cuts to subdivide the surface into the desired pattern which may be arranged to have terminals at opposite ends or the same end or in other desired configuration by scoring the film in any convenient manner with a knife. or by stamping, dinking or other such procedure. It is not imperative that the cuts pass entirely through the film as natural shrinkage on firing will tend to open the cut and, furthermore, the first passage of current will burn out weak connections leaving the reticulation or filigree of metallic conductor intact in those parts of the film which were uncut. Consequently cuts may be made in the film while retaining abacking sheet to provide support. Cuts can also be so made as to providea shape which will be readily fitted to a vesselhaving a nonplanar surface by cutting away certain parts and scoring other parts. This procedure enables application of integral heating elements, for example, to 10 l5 cm. rectangular dishes 2.5 cm. deep used in serving food in air flights. It is also possible to defer making cuts until the entire sheet has been affixed to a base.

Adhesion of the film to a base is conveniently effected using a small amount of the organic binder in the same solvent as an adhesive. Thus in preparing heating elements for constructing the drawing, approximately 7.5X12.5 cm. pieces of film on silicone-impregnated paper are applied to 10x15 cm. pieces of porcelain-enamelled steel using a solution of polyisobutylene in toluene (about 10 percent by weight) as adhesive followed by scoring with lines at 6 mm. intervals starting 6 mm. inside the left or right margin alternately and running to the opposite 12.5 cm. side to give a serpentine or meandered pattern with 20 legs each 6.3 cm. long so that the total pattern is about 125 cm. It will be recognized that 6 mm. at each side is not calculated in the leg. Elements of this size are suitable for application to the above-described rectangular dishes.

It will be recognized that electrical insulation must be maintained between the heating element and the steel or metal of the vessel. A flat steel sheet as described above would be a utensil for certain purposes, as for example, warming a biscuit. The insulation as provided above is the porcelain enamel which desirably is the same as that employed in compounding the heating elements film. The final utensil thus has a metallic shell, a porcelain enamel insulating layer and an integrally bonded heating element. Other insulative materials may be employed and, because the temperatures to be employed are generally not very high (frequently below C. during operation is heating elements and rarely up to 200 C. except when especially electrically resistant undercoatings are used), it is not necessary to limit compositions to the extent required for high-temperature insulative glazes and ceramics.

Firing of the assembled utensil and film proceeds in two stages. During the first stage the binder is depolymerized leaving the unsintered frit and silver or silver oxide as a residue. This is accomplished readily by firing the above specimens up to about 600 C. during 15 minutes. This is followed immediately by firing to the maturing temperature of the particular porcelain enamel employed, e.g., 800 C. for 3 minutes. Prolonged heating at higher temperatures may result in greater flow of the enamel and is therefore generally avoided. Firing is for a sufficient time to effect maturation of the enamel including recrystallization which may occur normally or even to some extent at a lower temperature stage.

During that stage of firing when there is no binder present, a heating element which is not lying flat may tend to drop away or spall. It is found that such undesirable effects are conveniently counteracted by employing a small amount (110 percent) of a low-temperature glass, such as a lead borosilicate glass, as fine particles in the polymer-solvent adhesive combination. This low-temperature glass is preferably one which melts below the temperature at which the polymer is entirely decomposed, for example, 300 to 500 C. The small amount involved is sufficient to adhere the heating element film without fluxing the silver so that the latter will flow. The silver is thus embedded in a substantially nonfluxing matrix.

As noted above, it is convenient to provide contacts to the heating elements of the invention by application to the terminals of a more metal-rich composition. This is done before firing so that all firing operations are conducted at once. A small piece of film or wire or a small amount of a paste of the desired metal-rich composition is applied wherever terminals or taps are desired. Moistening with solventand slight pressure is sufficient to assure contact. Leads can be connected to the heating elements in whatever manner or by whatever means is then compatible with the equipment being employed. For example, when using a heating element which is 65 percent porcelain frits and 35 percent silver oxide, contact spots are applied using a film which contains equal weights of frits and silver oxide. After firing the film which was originally 0.6 mm. thick has shrunk to about 0.l to 0.15 mm. thick and is thicker where the metal-rich spot is provided. For measuring resistances, contact is made using metal strips held in place by spring clips or wire leads are soldered to the metal-rich tab or spot.

What is claimed is:

l A film adapted for application of a conductive pattern to a utensil consisting essentially of depolymerizable organic binder, very finely divided metallic or metal-yielding conductive powder and recrystallizing porcelain enamel frit having particle sizes substantially five or more times as large as those of said conductive powder in a range of sizes between about 1 micron and about 100 microns.

2. A film according to claim 1 wherein the recrystallizing porcelain enamel frit has particle sizes predominately falling within the ranges in which the average diameter is from 5 to times the average diameter of the particles of conductive powder.

3. A film according to claim 2 in which the metal-yielding conductive powder is silver oxide with particle size of about 0.5 to 2 microns, the organic binder is polyisobutylene and the porcelain enamel frit is composed of recrystallizing frit containing titania and is predominately in two sizes of 1-5 microns and 50-105 microns.

4. A composition adapted to firing to conductive patterns comprising recrystallizing porcelain enamel frit having particles in a range of diameters predominately falling within the range of from about 5-10 up to 50-100 microns and silver oxide having an average diameter of about 0.5 to 2 microns.

5. A process for production of electrically heated utensil comprising the step of adhering to a utensil coated with porcelain enamel a coating comprising recrystallizing porcelain enamel frit in proportions of from about to about 50 percent of frit and silver oxide in proportions of about 15 to about 50 percent.

6. A process according to claim 5 wherein the coating is applied in the form of a film containing depolymerizable organic binder.

7. A process according to claim 6 wherein adhesion of the film is enhanced by application of an adhesive of depolymerizable organic binder in solvent therefor containing 1-1 0 percent by weight of low-melting lead borosilicate glass.

8. A process according to claim 7 wherein connection spots are provided by application of metal-rich film over portions of the film where contacts are desired.

9. As an article of manufacture the combination of a porcelain enameled utensil and an electrically conductive heating element, the fired product of claim 1, bonded to said utensil as an overlay embedded in the porcelain enamel thereof.

10. A film according to claim 1 in combination with a supporting backing. 

2. A film according to claim 1 wherein the recrystallizing porcelain enamel frit has particle sizes predominately falling within the ranges in which the average diameter is from 5 to 10 times the average diameter of the particles of conductive powder.
 3. A film according to claim 2 in which the metal-yielding conductive powder is silver oxide with particle size of about 0.5 to 2 microns, the organic binder is polyisobutylene and the porcelain enamel frit iS composed of recrystallizing frit containing titania and is predominately in two sizes of 1-5 microns and 50-105 microns.
 4. A composition adapted to firing to conductive patterns comprising recrystallizing porcelain enamel frit having particles in a range of diameters predominately falling within the range of from about 5-10 up to 50-100 microns and silver oxide having an average diameter of about 0.5 to 2 microns.
 5. A process for production of electrically heated utensil comprising the step of adhering to a utensil coated with porcelain enamel a coating comprising recrystallizing porcelain enamel frit in proportions of from about 85 to about 50 percent of frit and silver oxide in proportions of about 15 to about 50 percent.
 6. A process according to claim 5 wherein the coating is applied in the form of a film containing depolymerizable organic binder.
 7. A process according to claim 6 wherein adhesion of the film is enhanced by application of an adhesive of depolymerizable organic binder in solvent therefor containing 1-10 percent by weight of low-melting lead borosilicate glass.
 8. A process according to claim 7 wherein connection spots are provided by application of metal-rich film over portions of the film where contacts are desired.
 9. As an article of manufacture the combination of a porcelain enameled utensil and an electrically conductive heating element, the fired product of claim 1, bonded to said utensil as an overlay embedded in the porcelain enamel thereof.
 10. A film according to claim 1 in combination with a supporting backing. 